Cast iron annulus and method of manufacturing the same



Patented Apr. 3, 1934 UNITED STATES CAST IRON ANNULUS AND METHOD OFMANUFACTURING THE SAME Frederick C. Langenberg, Edgewater Park, andHerbert G. Reddick, Burlington, .N. J., assignors to United States Pipeand Foundry Company, Burlington, N. J., a corporation of New Jersey NoDrawing. Application August 20, 1932,

Serial No. 629,742

.8Claims.

Si. S. Mn. P.

w 3.00-3. 1.50-2.50 .o5-.1o .35-.80 .so-rs A typical example of an ironmixture ordinarily used for the casting of pipes in the way aboveindicated is as follows:

T.O. 0.0. St. S. Mu. P.

The object of our invention is, primarily, to provide an annealed,centrifugally cast cast iron pipe of materially increased ductility and,consequently, less brittleness than pipes as heretofore manufactured bythe described process from irons of compositions coming within the abovestated specification and to provide an improved method of manufacturingsuch pipes by means of which pipes of the improved quality andcharacteristics may be commercially produced.

Pipes as cast by the described method from irons coming within thespecification given above have usually what we might call a triplexcrystalline structure. In the outer stratum the structure is columnarand normal to the surface. In the middle stratum the structure isdendritic with more or less of a tendency of the dendrites to lie normalto the surface. In the inner stratum the structure is graphitic, thegraphite existing as medium sized plates or flakes. The three structureswill now be described in detail.

The principal component of the outer layer of this triplex metalstructure is, to the best of our present knowledge, an eutectic of ironcarbide 50 with austenite in various stages of decomposition throughwhich is interspersed fine particles of phosphide of iron;

The central area, which is dendritic, is composed of patches 'ofgraphite which tend to form nests in a matrix of silico ferrite,pearlite and phosphide eutectic. The dendritic structure graduallydisappears as the inside structure is approached. At the same time thephosphide eutectic becomes more pronounced and, when the insidestructure is reached, there exist plates of graphite in a matrix ofsilico ferrite, pearlite and phosphide eutectic. It should be understoodthat the demarc tion between the three structures is not clearly efinedbut there is a gradual transition from one representative structure tothe other. In castings, such as those described, it is possible, bychilling the metal very rapidly, particularly when silicon is in thelower limits of the range described, to practically eliminate the insideor graphitic structure. This is particularly true when the aboveconditions exist in conjunction with a thin casting. Under such acondition the outside structure and the structure described as thecentral structure remain unchanged in their characteristics.

Theexterior and middle structures previously described are characterizedafter casting by their hardness and brittleness, the outside structurebeing much harder and more brittle than the middle structure. It is alsoa well known fact and recognized in the arts that a cast iron castingmade by any process in which the cooling is relatively rapid has presenttherein internal strains which are often objectionable. I

In order to diminish the hardness and brittleness of the cast pipes aswell as to eliminate or modify any internal strains therein, it has beenthe practice to anneal the pipes substantially as described inthe patentto de Lavaud 1,280,418, the effect of this annealing treatment being tosubstantially eliminate the combined carbon, the result of which is todiminish the hardness of the outer and middle structure and to verygreatly increase the ductility of the pipe but while "pipes cast andannealedas above describedhave been made, marketed, and used in verylarge quantities and have given satisfactory service in use, it wouldobviously be very desirable if their ductility could be furtherincreased and we have discovered that this can be accomplished bysochanging the annealing conditions as to effect the elimination fromcombination of the carbon without at the same time bringing about thesolution of the iron phosphide and iron phosphide eutectic to more thana minimum degree and we believe we are the discoverers of the fact thatsuch castings can have their ductility materially increased if the ironphosphide and iron phosphide eutectic components of the casting are nottaken into solution during the annealing of the casting to a greaterextent than will bring about the presence in the finished casting of 65%or less of the phosphorus content of the casting in solution and,generally speaking, it is true that the smaller percentage of thephosphides dissolved in the ferrite the greater will be the ductility ofthe annealed casting. It must be understood that what we have saidrefers to annealed pipe in which the combined carbon has been reduced toa percentage of recognized tolerance, say, not to exceed 0.15% of themass of the casting.

While our experiments have shown that the higher the percentage ofundissolved iron phosphide and iron phosphide eutectic in the annealedcasting, the greater the degree of duc tility, it is not possible in anycycle of heating and cooling, such as cooling down a casting in the moldor of annealing, to avoid the passing into solution of some percentageof the undissolved iron prosphide and iron phosphide eutectic existingin the unannealed casting and the percentage unavoidably brought intosolution will vary with the thickness of the casting and with theduration of the heat treatment to which the ,casting is exposed and withthe temperature of the annealing heats which we have found effective toeliminate the combined carbon without at the same time bringing all oran unduly large portion of phosphides into solution. An annealed castingwhich preserves a considerable portion of its phosphorus as ironphosphide or iron phosphide eutectic constituent in undissolvedcondition will show material increase in ductility in comparison with asimilar annealed casting in which substantially all of the phosphideshave been dissolved. In one specific experiment it was observed that acasting having 37% of its phosphorus as iron phosphide or iron phosphideeutectic had a resistance of impact approximately twice as great as asimilar casting in which only 25% of its phosphorus as iron phosphide'oriron phosphide eutectic remained in the undissolved condition. In thecase of comparatively thin castings of, say, a pipe with a wallthickness of .34, we have found it quite practicable to substantiallyeliminate combined carbon from the casting without effecting thesolution of more than 50% of the phosphorus in the iron phosphide oriron phosphide eutectic constituent of the iron.

While it is true that the iron phosphide and iron phosphide eutecticpresent in undissolved form in a pipe centrifugally cast in a chilledmold from an iron coming within the specification which we have givenabove will dissolve to a greater or less extent during the process ofannealing at all temperatures above that of the critical point belowwhich carbon is not eliminated from its combination with the iron, wehave found that the rate of solution diminishes rapidly with decreasingtemperature and that by properly regulating the temperature to which thecast pipe is exposed during the process of annealing it is entirelypracticable to eliminate substantially all of the combined carbonwithout effecting the solution of more than 60% of the phosphorus in thephosphides. A practical an nealing process must, however, have dueregard to cost and time of treatment and these considerations, as wellas the elimination of the combined carbon and the maintenance in theannealed casting of not less than 35% of the phosphorus in thephosphides in undissolved condition. have been in our minds in devisingand perfecting our improved annealing method, which may be described asfollows.

We heat the pipes as rapidly as is practicable with avoidance ofbringing about such inequalities in temperature in different parts ofthe pipe as would produce injurious strains, to a temperature between1650 F. and 1725 F. In practice, we hold this high temperature atapproximately 1700 F. Heating to this temperature will in a short time,initiate the decomposition of even the most refractory carbides whichoccur in the outer stratum of the casting, thus making possible thesubstantially complete elimination of combined carbon from the castingby treatment at lower heats and within practically permissible timelimits. By limiting this high heat treatment to temperatures notexceeding 1725 F. we accomplish the necessary initiation ofdecomposition of the more refractory carbides without at the same timetaking in solution more of the iron phosphide and/ or iron phosphideeutectic components of the casting than is consistent with theproduction of a finished casting in which not more than 65% of thephosphorus will be found'in solution. Having effected the necessaryinitiation of decomposition of the refractory carbides, we then reducethe rate at which phosphorus is being taken into solution by reducingthe temperature of the casting as rapidly as practicable and at a ratewhich will not bring about dangerous inequalities of temperature indifferent parts of the casting, to a range of temperatures between 1450F. and 1350 F. and we hold the casting within this range of temperaturesfor a suflicient length of time to bring about the elimination ofcombined carbon down to a percentage not exceeding 0.15% of the mass ofthe casting. This practically concludes the annealing process, though wewould add that the pipe should be progressively cooled from the range oftemperatures between 1450" F. and 1350" F. to a temperature ofapproximately 1200 F. to avoid possible mischievous inequalities intemperature in different parts of the casting and after having reached atemperature of approximately 1200 F. a pipe can be cooled in air withoutmischievous results.

In developing our method as a practical process of manufacture, we haveannealed our pipes in a furnace of the general type illustrated anddescribed in the patent to Clark, Number 1,856,863, granted May 3, 1932.The length of the furnace with which our development work has been donewas 62 feet, 6 inches, and the furnace used, of course, supplied withoil burners and ventilating apertures for regulating the temperature ofthe different zones through which the pipes passed in moving from theentrance to the exit openings at the ends of the furnace. Operating uponpipes having the distinctive crystalline strata characteristics of pipescast in the manner previously described and of a diameter of 4 inchesand wall thickness of .32 inches and feeding the pipes to the annealingfurnace at a temperature approximating 1200" F., we find it practical toraise the temperature of the casting to approximately 1700 F. in 13minutes and to effect a suflicient initiation of decomposition in themore refractory carbides by holding the casting at or about thistemperature for a period of 5 minutes. The progressive cooling of thecasting from approximately 1700 F. to 1450" F. was satisfactorilyaccomplished in 7 minutes and by holding the casting point below 0.15%of the we: the casting. The progressive cooling of the pipe from 1350 F.

to 1200 F. was satisfactorily effected in 5 minutes and the pipes wereout injurious results.

Operating upon pipes of 24 inches in diameter and wall thickness of 0.8inches, the preliminary heating of the pipes from an entrancetemperature of 1200 F. to approximately 1700 F. required, under theconditions with which we were working, 27 minutes and to insure aninitiation of decomposition in the more refractory carbides we found itadvisable to maintain the casting at temperatures between 1650 F. and172'5" F. for a period of 10 minutes. The cooling from1650 F. to 1450 F.progressed through a period of 16 minutes and to effect a substantialelimination of combined carbon we held the pipes between 1450 F. and1350 F. for a period of 10 minutes and the cooling of the casting from1350 F. to 1200 F. was progressive through a period of 9 minutes.

Working with pipes of larger size and thicker then cooled in airwithwalls the times of exposure to the different heats tory results canbe obtained with the largest and heaviest castings by heating thecastings up to the range between 1650 F. and 1725 F. in a period not toexceed 40 minutes and by holding the castings between 1650 F. and 1725F. for a period not to exceed 15 minutes, then progressively cooling thecastings to 1450 F. through a period not to exceed 24 minutes and thenholding the casting at temperatures between 1450 F. and

1350 F. for a period not to exceed 16 minutes.

With very heavy pipes the period of cooling from 1350 F. to 1200 F. maybe as long as 14 minutes.

' From the facts given above, it will be obvious that our process forproducing from a cast pipe of the character specified, an annealed pipefrom which the combined carbon has been practically eliminated and whichwill not have dissolved phosphorus in excess of 65% of the totalphosphorous content of the metal, can be carried on under thoroughlypractical commercial conditions and that. the points to be had in viewfor the production of the best results, that is, of pipes containingdissolved phosphorus in the smallest possible percentage,- are, first,that the pipe should not be held at the range of temperature between1650 F. and 1725 F. for a longer period than that necessary toinitiatethe decomposition of the more refractory carbides; second, thatthe progressive heating of the pipes up to 1650 F. and the progressivecooling of the pipes from 1650" F. to approximately 1450 F. should becarried on as rapidly as practicable so as to avoid as much as possiblethe taking into solution-of phosphorm during these stages of theannealing pracess, and; third, that the casting should be held at thetemperatures between 1450 F. and 1350 F. only for such length of time aswill result in the elimination of combined carbon to a point notexceeding 0.15% of the mass of the casting. a

The distinguishing characteristics of our improved pipe are, in thefirst place, the composition of the iron of which it is formed, that isto say, composition coming within the specification given above, ofwhich the important considerations are that the phosphorus should exceed.30% and the silicon should not exceed 2.50%. The second distinguishingcharacteristic is that the annealed pipe preserves in outline at leastthe distinguishing crystalline structure which we have described aspeculiar to the castingas it comes from the mold. The thirddistinguishing characteristic is that our improved casting re tains inits structure not less than 35% of its phosphorous contents in the formof undissolved 8 iron phosphide and/or iron phosphide eutectic. Thefourth distinguishing characteristic is that our finished casting, whilecontaining not more than 65% of its phosphorous content in solution, hasa combined carbon content not to exceed 0.15% of the mass of the castingand the fifth distinguishing characteristic is the material increase inductility of the casting as compared with annealed castings made from,similar iron compounds and treated by previously known and practisedannealing methods.

Apart from'the materially increased ductility of pipes manufactured byour improved process, the process has the imp artant advantage of havingthe tensile strength of the iron at a temperature under 1725" F., verymuch greater than is the case when the casting is heated to ahigherdegree, say to 1800 F. as was customary and this enables the pipe to behandled in the annealing furnace much more freely and with less pre-.

C. Si. S. Mn.

3.00-3.85 1.50-2.50 .05-.10 .3&.80

said annealed annulus having a wall structure made up of concentricannuli, each retaining in outline the distinctive crystalline formationsuccessively occurring at different distances from the outer surface ofthe annulus when irons coming within the above specification are cast inan externally cooled centrifugal metal mold, having,

not more than 65% of its phosphorous content in solution and having notless than 35% of its phosphorous content present in the form ofundissolved iron phosphide and/or iron phosphide eutectic, said annealedannulus having not more than 0.15% of combined carbon with itsuncoxnbined carbon in graphitic form.

2. An annealed cast iron annulus as called for in claim 1, in which thephosphorous component of the metal of the casting in solution does notexceed 60% of the total phosphorous component. 3. An annealed cast ironannulus as called for in claim 1, in which the phosphorous component ofthe metal of the casting in solution does not exceed 50% of the totalphosphorous component. 4. The method of manufacturing annealed,

centrifugally cast, cast iron annuli from iron compounds coming withinthe following specification: v j V 0. 'sl. '8. r.

which consists in casting said iron compound in the form of an annulusin an externally cooled metal centrifugal mold, thereby'producing a castanulus having a wall structure made up of concentric annuli, each havingthe distinctive crystalline structures successively occurring atdifferent distances from the outer surface of the annulus characteristicof annuli so cast and an annular casting in which the phosphorouscomponent of the metal is, to a large extent, present in the casting inthe form of iron phosphide and/or iron phosphide eutectic; thenannealing said cast annulus by raising its temperature to a point above1650 F. and not to exceed 1725 F., for a sufiicient time to initiate butnot complete the decomposition of even the most refractory carbideconstituents of the casting, then progressively reducing the temperatureof the casting until said temperature reaches a range between 1450" F.and 1350 F., to reduce the rate at which phosphorus is taken intosolution and then holding the casting within said last mentioned rangeof temperatures until the combined carbon does not exceed 0.15% of themass of the casting and then progressively reducing the temperature ofthe casting, so as to avoid annulus to the range of temperatures between1650 F. and 1725 F. in a period not to exceed 40 minutes.

6. In the method of manufacturing annealed, centrifugally cast, castiron annuli, as called for in claim 4, the step which consists inmaintaining the annulus within a range of temperature between 1650 F.and 1725 F. for a period not to exceed 15 minutes.

7. In.the method of manufacturing annealed, centrifugally cast, castiron annuli, as called for in claim 4, the step which consists inprogressively lowering thetemperature of the annulus from thetemperature of 1650 F. to a temperature of 1450 F. in a period not toexceed 24 m1nutes.

8. In the method of manufacturing annealed, centrifugally cast, castiron annuli, as called for in claim 4, the steps which consist inprogressively raising the temperature of the annulus to the range oftemperatures at or above 1650 F. in a "period not to exceed 40 minutes,holding the annulus within a range of temperatures from' 1650 F. to 1725F. for a period not to exceed 15 minutes, progressively reducing thetemperature from a point at or above 1650" F. to a point within therangeof 1450 F. and 1350 F. during a period not to exceed 24 minutes.

- FREDERICK C. LANGEN'BERG.

HERBERT G. REDDICK.

